Cut slope angle, based on Friction Angle? 1

[BurgoEng]

I need to figure out the angle that a slope can be cut. the only info I have on the soil is a friction angle = 28deg, and unit weight of 125pcf. We are planning on excavating a large portion of a backyard to remove contaminated soil. I am tasked with determining the cut slope angle for safe excavation (total depth about 15ft). I know conservative estimates are usually a 1v:1h slope (45 deg), but with a friction angle of 28deg, is a 45 slope more conservative (ie is phi measured from the vertical) or less conservative (ie is phi measure from the horizontal.

Do I need to do a full blown soil stability analysis? What is the relation between friction angle and angle of repose....I can't seem to find that info anywhere.

These may sound like a silly questions, but I don't pretend to be a geotech, and soils was never my strongpoint in school.

 

[TDAA]

Well, if that 28 degree material is a sand with no cohesion, and you are in the US, OSHA will require a 1.5:1 (H:V).

If your "backyard" has a structure next to it, I would definately talk with a geotech. You could also have issues with seepage and other sorts of strength reducing issues. No sense in taking the liability of a soils mechanics problem if you are not a trained geotech.

 

[BigH]

On the other hand, if the soil is clayey, it is the undrained strength that governs and firm to stiff soils can stand vertical for a critical depth based on the undrained strength.
I don't think that I would put anyone in jeopardy in the excavation without a geotechnical engineer taking a look at this. What you don't need is any kind of failure. Depending on the depth of excavation to areal extent, you may require a "competent person" that will be responsible for the excavation.

 

[PEinc]

TDAA, good answer. Also, OSHA will want 1.5H:1V or flatter.

 

[PEinc]

P.S. - the 125 pcf and 28 degrees sounds like they were pulled out of the air. Was any testing done to determine or verify these properties? I doubt it. It's amazing how often soils reports say to use 28 to 30 degrees and 120 to 125 pcf. 125 pcf is pretty heavy for a granular soil with only 28 degrees. Consider getting real properties for a slope stability analysis.

 

[TDAA]

Thank you PEinc, that should have said or flatter. Also, that applies for a temporary cut slope, not a permanent. For final grading, if the area will remain cut, a flatter slope should be used for an adequate factor of safety.

The 28-degree may not be that far off, depending on the angularity of the soil particles. I have actually had some relatively clean sands with a measured Phi around 25. I almost threw out the results, but upon further inspection found the sands to be rounded, giving the lower than expected results. I agree though, they were likely estimated.

Burgoeng, the phi angle is measured form the horizontal; however, not on the soil directly. The value is measured off of a few points on a normal stress vs. shear stress plot, derived from shear testing. To answer your question though, the 45 degree slope is less conservative than something such as a 3:1(H:V) (or about 18.4 degrees).

 

[BurgoEng]

Thanks for the help everyone. The 28deg I was told for the soil was given to me based on a sieve test of some samples. It also said coheision = 0 (i guess I should have typed that at the start). From the % passing results, i guess it seems like it is a sand, as less than 25% passed the #200 sieve.

I'll be showing on my sketches to excavate at 2h:1V slope and any needed vertical cuts shall require bracing and shoring to be provided by the contractor on-site. This excavation is to be temporary (removing contaminated soils).

 

[blueoak]

I really think TDAA and BigH have the best advice in hiring a geotech. All the samples and models are based on a tiny sample that is only a best try at getting something worthwhile out of the ground. I like a geotech on site or to have seen the site, just because things change and it is nice to have the competent person to say if you can or can't do something. I haven't done contaminated soil removal, but I would guess you may want that flexibility during excavation. Also the cost savings of not going to a 2:1 or flatter may pay for the geotech, let alone their advice.

 

[PEinc]

BurgoEng,

Like I said, the 28 degrees was pulled out of the air. On most jobs I see, no testing is done other than sieve analyses and Atterberg Limits. Sieve analyses do not give soil friction angles. They may help estimate the angle, but do not actually calculate the angle. A sieve analysis also will not give cohesion. The report you have has estimated values which are usually on the very conservative side.

 

[oldestguy]

For what it is worth, on some OSHA type jobs, after a citation, I measured the angle of repose. Then calling for slightly lesser slope angle, and considering a very short duration of cut, OSHA lets the guy alone. No failures either. This was usually in clean sand, maybe 2 percent P-200, some moisture to give a little "apparent cohesion".

In my experience, an inexperienced geotech on the job is next to worthless if they are not backed up with an experienced person. As BigH has said in the past, get the most experienced engineer there, not a beginner alone.

 

[MRM]

I like oldestguy's suggestion about measuring the angle of repose. It's a very inexpensive way to determine a conservative/lower bound/loose friction angle value.

TDAA had some good comments about particle shape too. The degree of angularity certainly plays an important role in the behavior of sand; both for strength and compressibility purposes. Along with a hand lens to visually observe the grains, an angle of repose test can also be run to help determine overall grain shape if benchmark tests on similar gradations are already known.

 

[drobi]

The critical factor you have to assess is what is the risk of undertaking the re-grading of the backyard. If a failure of the slope would have significant effects on adjacent structures you would need to adopt a high factor of safety on any assessment. If local failure of the cut slope would be of no consequence the factor of safety could be significantly reduced.

From the soil parameters provided it would appear we are dealing with a loose sand.

The factor of saftey against a translational failure could be taken as tan of angle of internal friction divided by tan of angle of slope.

Or in this case, tan of angle of slope equals tan 28deg divided by FoS.

Assuming a FoS of 1.25 the safe angle of the slope would be 23deg